1. Field of the Invention
The present invention relates to a sorption vessel having an internal interstage screening system. The sorption vessel of this invention has particular utility for sorbing gold values from a slurry containing the same.
2. Description of the Background Art
Precious metals, such as gold and silver, and other metals, such as copper, iron and nickel, are typically contained in ore materials when mined. Metal-containing ores are typically processed in accordance with one or more known techniques so that the metals, especially precious metals, can be separated and extracted from the mined ore.
One known method of recovering precious metals or other metals from metal-containing ores occurs in a carbon-in-pulp system wherein activated carbon, usually in the form of activated charcoal, is mixed with a slurry of the ore in a cyanide solution. The carbon remains in contact with the slurry for a time sufficient to permit the gold and silver to become adsorbed by the carbon and, thereafter, the carbon is separated from the residue, typically by some type of interstage screen assembly. The carbon particles are generally larger than the finely ground ore particles which permits the screening step to be accomplished with relative ease. A carbon-in-pulp system utilizes a plurality of mechanically or pneumatically agitated tanks arranged in series, usually 4 to 6. Each tank generally contains activated carbon having a different amount of gold adsorbed thereon; with the first tank having the highest and the last tank having the lowest. A slurry of a finely ground ore and the alkaline cyanide metal complex solution, is introduced into the first tank, while the carbon is advanced countercurrently to the flow of slurry from the last tank to the first tank. The slurry is agitated with the carbon adsorbent in the tank and the carbon adsorbs the cyanide metal complex as the slurry and the carbon adsorbent, i.e., the pulp, is agitated. The pulp is sequentially passed through the series of the pneumatically or mechanically agitated tanks so that most of the cyanide metal complex is adsorbed by the carbon.
After passing through the series of tanks, the processed tailings are discarded. As stated above, the carbon containing the adsorbed cyanide metal complex can be sequenced through the tanks in reverse order from the ore slurry. After the adsorbent has passed completely through the system, it has become "loaded" with the adsorbed metal complex. The loaded adsorbent is then chemically processed to remove most of the metal. The stripped adsorbent is then reactivated and then returned to the carbon-in-pulp system.
Another known method of recovering metals from metal-containing ores occurs in a carbon-in-leach system. The carbon-in-leach system is similar to the carbon-in-pulp system. The primary difference between the carbon-in-pulp system and carbon-in-leach system is that in the former, there are mixing tanks for cyanidation leaching prior to the carbon adsorption stage. In a carbon-in-leach system, cyanidation is conducted in the presence of carbon.
A further method of recovering metals from metal-containing ores occurs in a resin-in-pulp system. Generally, in a resin in pulp system, a leached metal-containing ore pulp is exposed to a resin, typically employed as moderately coarse particles, in a series of agitator tanks. The particular resin is, thereafter, separated from the pulp with the metal complex adsorbed thereon and, ultimately, the metal complex is removed from the resin to recover quantities of the particular metal present in the ore.
A detailed review of the screening systems currently in use for separating either the carbon or resin adsorbents from the slurry of a pulp can be found in the following art: P. A. Laxen, "Interstage Screens On The Adsorption Circuit of An `In-Pulp` Process" and Gold & Silver Recovery Innovations, "CIP Interstage Screens", Phase III, Vol. 7, Ch. 42, pp. 4079-4184. Initial carbon-in-pulp ("CIP") plants utilized external vibrating screens over which the pulp with entrained carbon was pumped by air lifts from the bottom of the adsorption tanks. Each tank utilized a number of external vibrating screens to which external air lifts on the side of each tank lifted pulp plus carbon onto the screens. The screen pulp would flow by gravity to the next tank while the carbon on the screen flows back to the tank from which it came except periodically when it is diverted to the next tank countercurrent to the pulp flow. The disadvantages associated with this type of system include (1) the capital cost of external vibrating screens, air lifts and compressors for air supply; (2 ) additional costs of supporting structures for the screens; (3) compressed air and screen maintenance are relatively high for a large scale plant with the system being much more energy intensive than it needs to be; (4) the large amount of air injected into the pulp results in substantial carbon scaling; (5) the efficiency of the system is low since a portion of the carbon is continuously not in contact with pulp and therefore not absorbing metal values therefrom; and (6) excessive operator manpower required on large plants for monitoring the system for ruptures and/or replacement of the screen media.
To upset these disadvantages, systems have been installed which utilize both external and internal screens, with the external screens being used for carbon transfer only. The majority of the pulp (about two-thirds) flows to internal air cleaned screens while the remainder of the pulp is continuously pumped by a submergible pump to the external screens with the carbon either being returned to the leach tanks or transferred to the next tank as required for countercurrent carbon transfer. The pulp from the external screen, again one-third of the total, flows continuously to the next tank.
In about 1982, a new type of screen evolved, these known has the EPAC (equal pressure air cleaned) screens. By damming the pulp flow on the downstream side of the screen, the hydrostatic pressure is equalized on both sides of the screen and is not as readily blinded by carbon particles pinned to the screen surface. This simple technique increases the screen capacity of a screen panel per unit of length by a factor of 10 or more. An illustration of this type of screen is shown in U.S. Pat. No. 2,808,928. A wide variety of types, sizes and configurations of EPAC screens is known in the prior art. Each of these systems has its own attendant disadvantages mostly relating to the difficulties in keeping the screen clean of carbon build up along with the difficulties of achieving high throughput. In addition, the quantities of air required and the operational aspect for use of such air create further disadvantages for this type of system.
One improvement on the EPAC screens is known as KAMBALDA screens. These screens were able to dispense with the air cleaning system by mounting the screen horizontally in an upper portion of the tank and by installing an agitator blade beneath the screen in a manner such that pulp is directed against the screen with the agitation caused by the blades continually moving the carbon away from the screen to prevent build up. The screened pulp is removed from the top of the tank and is introduced into the subsequent tank in the lower portion thereof with the mixture in the second tank causing the pulp to flow upwardly and onto the next screen. The agitator blade is known as a sweeper arm and is mounted a few inches away from the bottom of the screen. While the total energy requirements for this type of screen is less than conventional EPAC screens, the disadvantages for this system relate to the extensive amount of structural steel required above the tank to support the agitator with its sweeper, as well as the screens located thereabove.
Another improvement on the EPAC screens was made at North Kalgurli Mines and is known as the NORKAL screen. This screen consist of a cylindrical screen basket along with pedals which rotate along its circumference to sweep away carbon build up. Screened pulp flowing into the cylindrical screen basket is drained through an out-flow and is directed to the next tank. The improvement of this type of arrangement relates to an increase in throughput with a corresponding decrease in energy necessary to achieve that throughput. The disadvantages of such a system are similar to those identified above for the KAMBALDA screens.
As noted in the articles cited above, the shortcomings associated with known interstage screening installations are their energy intensive operational requirements, which are predominantly ascribed to the compressed air requirements; the high maintenance and operational costs, since, in order to advance the adsorptive species on a continuous or semi-continuous basis, as is necessary, the internal launder or mechanical interstage screening system must pump the total pulp (i.e., slurry and carbon); the low throughput per unit area; and the absence of any separation or concentration of the adsorptive species from the pulp. Additionally, due to the abrasive nature of the adsorptive species, the conventional wire or cloth interstage screens have a relatively short effective life which necessitates constant inspection and replacement, else the adsorptive species and, hence, the adsorbed metals, remain in the pulp and cannot be recovered.
Other problems associated with prior art installations relate to carbon transfer pumps used in such installations and, more specifically, the deleterious effect they have on the sorbent.